Electrolytic cells comprising ceramic membranes that selectively transport ions are known in the art. By having an ion-selective membrane in the electrolytic cell, certain ions are allowed to pass between the cell's anolyte compartment and catholyte compartment while other chemicals are maintained in their original compartments. Thus, through the use of an ion-specific membrane, an electrolytic cell can be engineered to be more efficient and to produce different chemical reactions than would otherwise occur without the membrane.
These ion-selective membranes can be selective to either anions or cations. Moreover, some cation-selective membranes are capable of selectively transporting alkali cations. By way of example, NaSICON (Na Super Ion CONducting) membranes selectively transport sodium cations, while LiSICON (Li Super Ion CONducting) and KSICON (K Super Ion CONducting) membranes selectively transport lithium and potassium cations, respectively.
Electrolytic cells comprising alkali cation-selective membranes are used to produce a variety of different chemicals and to perform various chemical processes. In some cases, such electrolytic cells convert alkali salts into their corresponding acids. In other cases, such electrolytic cells may also be used to separate alkali metals from mixed alkali salts. One non-limiting example of a conventional 2 compartment electrolytic cell 10 is illustrated in FIG. 1. Specifically, FIG. 1 illustrates the cell 10 comprises an anolyte compartment 12 and a catholyte compartment 14 that are separated by a NaSICON membrane 16.
During operation, the anolyte compartment 12 comprises an aqueous sodium-salt solution (NaX, wherein X comprises an anion capable of combining with a sodium cation to form a salt) and current is passed between an anode 18 and a cathode 20. Additionally, FIG. 1 shows that as the cell 10 operates, water (H2O) is split at the anode 18 to form oxygen gas (O2) and protons (H+) through the reaction 2H2O→O2+4H++4e−. FIG. 1 further shows that the sodium salt NaX in the anolyte is split (according to the reaction 4NaX+4H+→4HX+4Na+) to (a) allow sodium cations (Na+) to be transported through the NaSICON membrane 16 into the catholyte compartment 14 and (b) to allow anions (X−) to combine with protons to form an acid (HX) that corresponds to the original sodium salt.
The above-mentioned electrolytic cell may be modified for use with other alkali metals and acids corresponding to the alkali salts used in the anolyte. Moreover, it will be appreciated that other electrolytic reactions may occur which result in proton formation and corresponding lowering of pH within the anolyte compartment. Low pH anolyte solutions in such electrolytic cells have shortcomings. In one example, at lower pH, such as a pH less than about 5, certain alkali conducting ceramic membranes, such as NaSICON-type membranes, become less efficient or unable to transport sodium cations. Accordingly, as the electrolytic cell operates and acid is produced in the anolyte compartment, the cell becomes less efficient or even inoperable. In another example, acid produced in the anolyte compartment can actually damage the NaSICON membrane and thereby shorten its useful lifespan.